1. Field of the Invention
The invention relates to a gas separation and more particularly, it relates to the enhanced production of oxygen from air.
2. Description of the Prior Art
Cryogenic separation of air requires a pre-purification step to remove contaminants such as water, CO2 and hydrocarbons from air. In cold sections of the separation process (such as heat exchangers and the main condenser), water and CO2 can solidify and block the heat exchangers or other components in the distillation columns. Acetylene and other hydrocarbons in air present a potential hazard. The high boiling hydrocarbons can accumulate in the liquid oxygen and create an explosion hazard. Thus, those impurities in air must be removed prior to the cryogenic distillation of air. One method to remove the air contaminates is to use an adsorptive process on the pressurized feed air stream. The prepurification system removes the above impurities using a thermal swing (TSA) or a pressure swing (PSA) adsorption process. There is always a desire to reduce the cost of the prepurification process through improved adsorbents, process or equipment modifications. An improvement in the PSA process through the introduction of a regenerator in the adsorption bed in order to control the internal bed temperatures is desired to achieve improved adsorption of CO2, N2O, and hydrocarbons and also better regeneration of the water adsorbent.
It is now believed that nitrous oxide (N2O) also should be removed from air prior to separation. N2O is present in air at a concentration of about 300-375 ppb. The presence of N2O can be a serious problem for cryogenic air separation units (ASU) because it can form solid deposits in the primary heat exchangers or the main condenser of the distillation system. This can result in degraded performance and can even cause blockage of heat exchangers. Therefore, to avoid these problems, it is advisable to remove N2O to a level below about 50 ppb prior to the cold box in cryogenic air separation units. Wenning (“Nitrous Oxide in Air Separation Plants” Proceedings from MUST 1996, pp. 79-89) has described this problem in detail. N2O is inert in liquid oxygen, however, it can become quasi-permanently present in the distillation column and potentially freeze there unless removed by costly periodic liquid oxygen drainage. N2O also decreases the solubility of CO2 in liquid oxygen, thereby increasing the potential for freezing of CO2 in the distillation columns.
Air prepurification can be accomplished using pressure swing adsorption (PSA), temperature swing adsorption (TSA) or a combination of both (TSA/PSA) incorporating either a single adsorbent or multiple adsorbents. When more than one adsorbent is used, the adsorbents may be configured as discrete layers, as mixtures, composites or combinations of these. Impurities such as H2O and CO2 are commonly removed from air using two adsorbent layers in a combined TSA/PSA process. Normally, a first layer of activated alumina is used for water removal and a second layer of 13X molecular sieve is used for CO2 removal. Prior art, such as U.S. Pat. No. 4,711,645, teaches the use of various adsorbents and methods for removal of CO2 and water vapor from air in a PSA process.
U.S. Pat. No. 5,169,413 relates to pressure swing adsorption gas separation gas operations that are enhanced by the retention and use of internal refrigeration effects, without the need for externally supplied refrigeration.
U.S. Pat. No. 5,674,311 relates to an adsorption process and system for the selective adsorption of a more readily adsorbable component, such as nitrogen, as from air or other feed gas mixture is carried out using a composite adsorbent bed containing different adsorbent material positioned in separate zones in which the temperature conditions favor adsorption performance of the particular adsorbent material under applicable processing conditions in each zone. A method for the selection of the adsorbent materials is based on Adsorption Figure of Merit values.
U.S. Pat. No. 5,989,314 relates to a pressure swing adsorption air prepurifier that is used to remove water, carbon dioxide and hydrocarbons from a feed gas stream, such as a feed air stream, passing to a cryogenic air separation plant. By the incorporation of a regenerative heat exchange as an integral part of the air prepurifier, the cooling effects of the desorption of water are stored and transferred so as to cool the incoming feed air stream passing to the adsorbent material within the air prepurifier. The productive capacity of the adsorbent material is enhanced thereby.
U.S. Pat. No. 4,472,178 relates to a process that is set forth for the removal of carbon dioxide from air in an adsorption bed wherein the regeneration energy is reduced by the use of a heat recuperator and a purge sequence which avoids the removal of heat from the adsorption bed during regeneration.
Izumi, Jun, “High Efficiency Oxygen Separation With The Low Temperature and Low Pressure PSA,” November 1989, pp. 1-10, A. I. Ch. E., San Francisco, Calif. discusses the use of a regenerator section after the water removal section and before the nitrogen removal section of an O2 VPSA system.
The design of a regenerator is known in the published literature. Two such references are:
Kays, W. M. and London, A. L., “Compact Heat Exchangers”, 2nd ed., McGraw-Hill, New York, 1964.
Furnas, C. C., “Heat Transfer From a Gas Stream to a Bed of Broken Solids,” Ind. Eng. Chem., Vol. 22 1930.
An objective of the present invention is to improve the performance of a PSA, TSA or PSA/TSA prepurifier to remove contaminates such as CO2 and N2O from the air stream prior to cryogenic separation. Preferable, a method of this invention can be used to improve the performance of any PSA gas separation process where there are large temperature changes in the bed due to the adsorption of one or more of the gas species in the feed stream. Such species could be water vapor, CO2, ammonia, hydrogen sulfide, sulfur dioxide, etc. Examples of these processes are CO2 removal and drying of natural gas, and CO2 removal and drying of syngas streams or in hydrogen PSA.
It is another object of the invention to provide an improved PSA process and apparatus for the production of oxygen from air, and other desirable gas separations.
It is a further object of the invention to provide a PSA process and system for enhancing the overall efficiency and economy of oxygen production from feed air.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.